Fuel cell system control

ABSTRACT

A fuel cell system is controlled responsive to the hydrogen partial pressure in the anode effluent and the reformer-reactor temperature. The control concerns regulating stream flow to a venturi mixer dependent upon the hydrogen partial pressure. Auxiliary fuel flow may be scheduled responsive to abrupt changes in gross current to eliminate the time lapse normally associated with flow of fuel from the reformer to the fuel cell. Supplemental fuel is supplied to the reformer-burner dependent upon reactor temperature.

O United States Patent 1 3,607,419

[72] Inventor Stephen J. Keating, Jr. [56] References Cited [21] A I Noglgggg UNITED STATES PATENTS o Parker CI 3]. patented s p 21 19713,516,807 6/i970 W851 6! 8i. 136/86 [73] Assignee United AircraftCorporation Primary Examiner-Winstn A. Douglas East Hartford, Conn.Assistant Examiner-11. A. Feeley Attorney-Edmund C. Meisinger ABSTRACT:A fuel cell system is controlled responsive to the hydrogen partialpressure in the anode effluent and the reformer-reactor temperature. Thecontrol concerns regulating stream flow to a venturi miixer dependentupon the [54] FUEL CELL SYSTEM CONTROL hydrogen partial pressure.Auxiliary fuel flow may be 2 Claims lDnwin Fi scheduled responsive toabrupt changes in gross current to g eliminate the time lapse normallyassociated with flow of fuel U-S. from [he reformer {o the fuel celLSupplemental fuel is up. [i] lnt.Cl ..H0lm27/l2 plied to (hereformer-burner dependent upon reactor lem- [50] Field Of Search Bperature EXHAUST 38; /8; ""V'VW vv' lL PRE-HEATER BOILER \"I'I Jy'v'VvV-Z0 OAD M /Z L g r', SHIFT CONV REACTOR 1 l vt'vv'vv'fr BURNER I z 44 3113: 36 g M AIR- E 52 SUPPLEMENTAL FUEL FUEL CELL SYSTEM CONTROLBACKGROUND OF THE INVENTION This invention pertains to a control forfuel cell systems. More specifically, this invention pertains tocontrolling the steam flow and resultant fuel flow to a reformer as afunction of a selected system variable and maintaining reactortemperature.

Generally, pure hydrogen has been recognized as the preferred fuel forfuel cells and its coreactant has generally been oxygen or air. Fuelcells have been developed which utilize relatively impure hydrogen orother oxidizable fuels. Because of a desire to produce electric currentfrom relatively small powerplants, there has been considerable effortexpended to produce a system which is readily adaptable to operation onfuels commonly available. Various techniques have been proposed forconverting hydrocarbons and other hydrogen containing carbonaceousfeedstocks into hydrogen for use in fuel cells, but primary emphasis hasbeen placed upon steam reforming at relatively high temperatures.Typically, high-pressure steam reformed feedstock has been passedthrough palladium-silver separators to provide pure hydrogen for thefuel cell. In some fuel cells, for example the carbon dioxide tolerantcells and cells which use carbon monoxide tolerant catalysts, it ispossible to use a low-pressure reformer where no separation of hydrogenfrom the undesirable gases in the feedstock is necessary. In any ofthese systems, numerous techniques have been utilized in controlling thehydrogen reforming process in cooperation with the demands of the fuelcell. Control of these systems presents numerous problems, It is knownto monitor reactant pressures, humidity levels, electrolyteconcentration, flow rates, and a number of other parameters to keep thefuel cell system under optimum conditions.

The fuel cell is a demand system wherein electrochemical reactionsproceed at a moderate rate when the external load circuit is closed.Fuel must be supplied to the fuel cell so that current can continue tobe generated. It is axiomatic that the reforming process must be adaptedto conform to the demand requirements of the fuel cell.

SUMMARY OF THE INVENTION It is an object of this invention to provide animproved method of controlling a fuel cell system.

Another object of this invention is the regulation of the reformer steamand fuel feed responsive to the hydrogen consumption demands of the fuelcell while maintaining the reactor temperature. The system features theuse of a venturi-type mixer to maintain the fuel to steam ratio desiredin the reformer and to control the rate of flow of the mixture.

In accordance with this invention, hydrogen partial pressure in theanode exhaust from the fuel cell is sensed and used to control the steamflow through the venturi. As the primary steam flow in the venturiincreases, the secondary flow of fuel will similarly increase. Thesestreams mix and are supplied to the catalytic reactor section of thereformerwhere the fuel is steam reformed to provide hydrogen and otherresidual gases. Thereafter, this mixture is directed to the fuel cell.To improve transient response, reactor temperature is sensed and theflow of supplemental fuel to the reactor burner adjusted accordingly.

BRIEF DESCRIPTION OF THE DRAWING The single FIGURE is a schematic of afuel cell system embodying the control of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the FIGURE, a fuelcell stack is shown having output leads 12 through which electronsgenerated in the individual fuel cells are made available to theexternal load 14. Sensor 16 responds to fuel cell gross current. Wateris supplied to preheater 18 where the temperature of the water supplyincreases prior to entry into the boiler 20 where the water supply isconverted to steam. The steam is ducted to venturi 22 where fuel isaspirated thereinto by virtue of the flow of steam. Fuel is suppliedthrough conduit 24. The mixture of fuel and steam is directed to thereactor 26 portion of a reformer. In the reactor 26, fuel is steamreformed in the presence of a catalyst to the individual constituents ofthe fuel which typically are hydrogen, carbon dioxide, carbon monoxide,and certain residual water and methane. The steam reformed fuel isthereafter directed to a shift converter 30 where carbon monoxide reactswith residual water to produce carbon dioxide and additional hydrogen.This stream is passed through the heat exchanger 32 where heat issupplied to preheat the water supply. The hydrogen rich feedstockstream, at a decreased temperature, is directed through the anodecompartments of the fuel cell stack 10.

Ordinarily, more fuel is circulated through the cell than will beutilized in the fuel cell. It is generally desirable to design fuelcells to operate at a constant fuel utilization. Thus the hydrogenpartial pressure at the fuel cell exit ideally remains a constant. Theamount of utilization is determined in part by the excess fuel that mustbe left to go to the burner and thus to heat the reactor. The excess ofthe circulated fuel corresponds to the anode effluent and dischargesfrom the cell stack and is directed through heat exchanger 34 associatedwith the exothermic shift converter 30 where heat is added. The streamfrom the fuel cell is directed to the bu'mer 36 in the reformer wherethe stream is combusted with air from duct 28, to supply heat for thesteam reforming reaction which occurs in the reactor 26. The burnedexhaust gases exit through the heat exchanger 38 adjacent boiler 20.

The reformer fuel to steam ratio is maintained by the venturi mixer inwhich the steam flow is set proportional to demand by a valve 40. Thevalve 40 is controlled by regulator 42 which is responsive to a signalfrom the hydrogen partial pressure sensor 44. Hydrogen partial pressureis maintained constant at a predetermined level regardless of fuel cellload by increasing the feed flow to the reformer as hydrogen partialpressure decreases and decreasing the feed flow as hydrogen partialpressure increases. As steam flow increases by opening of the valve 40,additional fuel enters at the throat of the venturi due to the loweringof pressure there with increased flow. Auxiliary fuel can also beinjected at the venturi throat if required to handle sudden positivetransients. The sensing of sudden transients is conveniently determinedby the current sensor 16 and transmitted to regulator 48 which monitorsthe auxiliary fuel flow.

The fuel cell is a demand system. Consequently, the reformer mustreplenish the fuel used in the electrochemical reactions occuring withinthe fuel cell. The reformer feed is a mixture of steam and fuel and isscheduled to increase with decreasing hydrogen partial pressure in. theanode exit stream. If the fuel supply is below that required by both thefuel cell and the reformer burner, a condition which can occur duringpositive load transients, an insufficient amount of gas will exit fromthe cell stack and be burned in the reformer. Thus, reformer temperaturedecreases and reaction rate decreases accordingly. Sensor 46 detectsthis temperature change which is a departure from the normal operatingtemperature of the reactor and the control 50 converts this informationto a signal which operates valve 52 to provide supplemental fueldirectly to the burner to quickly bring it back up to correcttemperature. This is done to avoid the time delay which would otherwisebe encountered due to the need for the increased feed stock flow calledfor at the venturi valve 40 by the hydrogen partial pressure sensor 44to make its way around the system to the burner.

Summarizing, the system is designed so that as fuel cell effluenthydrogen partial pressure changes with load, fuel and steam feed flow tothe reformer is changed to bring the partial pressure back to apredetermined value. Use of a venturi-type fuel and water controlautomatically provides steam proportional to the fuel without the needfor metering either the fuel or the water supply. For example, no fuelpump is required if the proper height relationship is establishedbetween the fuel inlet in the venturi and the fuel tank. The venturi hasbeen shown with an auxiliary fuel supply to handle sudden large positivetransients which is controlled by a system sensing fuel cell outputcurrent. Supplemental fuel is provided directly to the burner tomaintain reactor temperature during sudden positive load transients whenthe increased feedstock flow would otherwise cool the reactor belownormal operating limits. This decouples fuel cell and reformer problemsduring transients and also during starting.

Although the invention has been shown and described with respect to apreferred embodiment, it should be understood by those skilled in theart that various changes and omissions in the form and detail may bemade therein without departing from the spirit and the scope of theinvention.

lclaim:

1. In a fuel cell system having a fuel cell stack consisting essentiallyof a plurality of individual cells in stacked relation, each cellincluding spaced oxidant and fuel electrodes, and electrolyte spacetherebetween, and chambers on the sides of the electrodes opposite theelectrolyte space for oxidant and fuel gas respectively and having areactor wherein a feed mixture of fuel and steam is steam reformed inthe presence of a catalyst with heat supplied by a burner therebyproducing a hydrogen containing fuel stream as a source of fuel for thefuel cell, the method of regulating the feed mixture to the reformercomprising:

maintaining fuel to steam ratio to the reactor by a venturi mixer wheresteam is the primary flow and fuel is aspirated thereinto; sensing thehydrogen partial pressure in the fuel cell anode exhaust and sensing thereactor-operating temperature; controlling the steam flow through theventuri mixer as a function of hydrogen partial pressure in the anodeexhaust; providing supplemental fuel flow to the burner during positiveload transients as a function of reactor temperature. 2. The methodclaim 1, including: injecting auxiliary fuel at the venturi throatresponsive to abrupt increases in fuel cell gross current.

2. The method claim 1, including: injecting auxiliary fuel at theventuri throat responsive to abrupt increases in fuel cell grosscurrent.